Projection system with additional series connected light valve for enhanced contrast

ABSTRACT

A projection system is provided that uses an additional light valve in series with the conventional color splitting-converging prism in order to increase the contrast of a projected image, wherein bit sequences are generated for the additional light valve that do not result in interference with the conventional color light valve bit sequences.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to projection systems, and moreparticularly to a projection system that uses an additional light valvein series with the conventional color splitting-converging prism, toincrease the contrast of a projected image.

2. Description of the Related Art

A typical color digital projector comprises a lamp, an illuminationsystem, and a light engine that includes a color splitting-recombiningprism (e.g. plumbicon prism). The optical function of the light engineis to split uniform illumination light into Red/Green/Blue (RGB)channels, relay each of the colors onto a light valve, such as a DMD(Digital Micromirror Device), and then re-combine all three channelsinto a single illumination light beam that is projected on a screen viaa projection lens.

The DMD is an electromechanical device consisting of millions ofmicroscopic mirrors that modulate light by independently flipping eachmirror through a predetermined angle. Using three such DMDs on the colorsplitting-recombining prism, a white light cone from the lamp isseparated into red, green, and blue channels. Each color is individuallymodulated by a respective DMD and then recombined by the prism.

The DMDs modulate the light by turning the mirrors on and off severaltimes during a video frame. A frame is divided into approximately 20 to60 bit planes of different duration, based on bit sequence and framerate. During a given bit plane each pixel on the screen is controlled bya single bit and is either driven ‘ON’ or ‘OFF’ for the entire durationof the plane. The number, duration, and location of the “ON” times areadjusted with respect to the timing of the frame for controlling thelight level. Combining the ‘ON’ times for a given pixel gives the pixelits proper intensity.

It is possible to add an additional light valve, such as a white DMD, inseries with the conventional color (RGB) DMDs in order to improve theimage contrast, without any of the compromises or artifacts that resultfrom the use of a dynamic iris. The additional light valve enhancescontrast by reducing the light incident on the color DMDs on apixel-by-pixel basis such that different intensity levels turn the lighton and off at different times. However, because a DMD modulates light byturning the mirrors ‘ON’ and ‘OFF’ over time in a series of bit planes,adding an additional light valve results in the intensity of a whitepixel being, in most cases, different than the intensity of one of theR, G or B values for that pixel (i.e. the white and color DMDs are ‘ON’and ‘OFF’ at unrelated times). This, in turn, results in unpredictablecolor and intensity variations and image artifacts.

Although the problem of unpredictable color and intensity variations maybe overcome somewhat by using certain different technologies for theadditional light valve (e.g. LCOS, LCD), other problems are introducedthrough the use of such different technologies. In particular, thesetechnologies operate so as to dim all of the light (i.e. not just theoff-state light), so that the input signal must be amplified in order tocompensate and bring the image light output back to its original level.The amplification must spatially match the white image. However, exactspatial matching is not possible in the presence of any convergenceerror whatsoever. Although the latter problem may be solved by softeningthe edges of the white image and the gain function applied to the image,the effective gain in contrast is thereby reduced. Also, the white imagecannot include any steep intensity slopes since these will be convertedto image artifacts by any convergence error. LCOS and LCD technologiesare also heat limited and do not function well in a high brightnessprojector.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a projection systemthat uses an additional light valve in series with the conventionalcolor splitting-converging prism in order to increase the contrast of aprojected image, wherein bit sequences are generated for the white DMDthat do not result in interference with the color DMD bit sequences.

This together with other aspects and advantages which will besubsequently apparent, resides in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE SOLE DRAWINGS

FIG. 1 is a schematic representation of a conventional color digitalprojector.

FIG. 2 is a schematic representation of a light engine of the projectorshown in FIG. 1.

FIG. 3 is a block diagram showing image processing modules for applyingimage data to the light engine of FIG. 2.

FIG. 4A is an electrical schematic representation of a light engineaccording to an embodiment of the invention, having an additional whiteDMD.

FIG. 4B is an optical schematic representation of a light engineaccording to the embodiment of FIG. 4B.

FIG. 5 is a block diagram showing how the bit planes for the white DMDare generated in the light engine of FIGS. 4A and 4B, according to anembodiment.

FIG. 6 is an exemplary timing diagram showing generation of white bitplane data from red, green and blue bit plane data using the white planegeneration module of FIG. 5.

FIG. 7A shows an example of green data for one bit plane, and FIG. 7Bshows an example of a resulting white bit plane where the white area isexpanded by one pixel and two pixels beyond the green data, using thewhite plane generation module of FIG. 5.

FIG. 8 is a block diagram of a white plane generation module of thelight engine shown in FIG. 4 with keystone correction of the light onthe white panel, according to an alternative embodiment.

FIG. 9A shows segmentation of a light imaging panel into segments, andFIG. 9B shows sequencing of bit planes for display of the segmentsdepicted in FIG. 9A, according to an embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a typical projector comprising a Xenon lamp and parabolicreflector 1 for creating a light cone 3 that passes through a UV filter4 into an illumination system 2. The lamp is located at a first focalpoint of the parabolic reflector 1 for re-imaging the light cone 3 at asecond focal point within the illumination system 2 that is co-incidentwith an integrator rod 12. The integrator rod converts the illuminationpattern output from the lamp to a uniform rectangular pattern. Therectangular light beam output from integrator rod 12 is then imaged ontoa light engine 5 by a relay lens system 13, and then projected by aprojection lens 14 onto a screen (not shown) to produce an image.

As shown in FIG. 2, light engine 5 includes a color splitting-convergingprism (typically a plumbicon prism) with three prism elements. The prismcomprises respective red channel 9, green channel 8 and blue channelsub-assemblies 7, each of which includes a light valve 10. The prismelements contain dichroic coatings 6 a, 6 b to separate the incomingwhite light into blue, green, and red. Each color is then separatelymodulated at each light valve 10. According to the exemplary embodiment,the light valves 10 are DMDs. As discussed above, a DMD is anelectromechanical device that typically consists of millions ofmicroscopic mirrors for modulating light by independently flipping eachmirror through a predetermined angle (e.g. +−12 degrees, although otherangles are possible). Each DMD 10 reflects the modulated light, which isre-converged by the prism into a colored image beam 11 for projection onthe screen.

With reference to FIG. 3, image pixel data (e.g. motion picture) isselected from a source input and measured at module 30, modified withinimage processing module 32, including scaling, warping, color matching,etc., and applied to a formatter that comprises a converter 34 forconverting the pixel data to binary bit planes The pixel data that getsconverted into bit planes, consists of a sequence of frames, eachcontaining an array of pixels comprising an image. The bit planes arethen loaded into a frame buffer 36 and thence to the DMDs 10.

The source of pixel data applied to converter 34 can be image processingmodule 32, or an image stored in memory, or data received over any videointerface, and the data may but need not be measured or processed usingmodule 30. If measurements are performed, they typically include thefrequency of the horizontal and vertical sync pulses, and often includethe location of the active window as well. The image processingfunctions set forth above are a few well known examples, and couldinclude fewer or many others, as would be understood by a person ofskill in the art.

FIGS. 4A and 4B are electrical and optical schematic representations,respectively, of a light engine according to an embodiment of theinvention, which includes an additional light valve, white DMD 10′, forcontrast enhancement, and a 4-color prism 43 with facets for each of thewhite DMD 10′ and colored DMDs 10. The prism 43 is functionallyillustrated in FIG. 4B as comprising a color splitter 43A and colorre-combiner 43B. Image pixel data is received from a source 40 (e.g.hard drive) and processed via electronics 41, which incorporate modules30 and 32 of FIG. 3. A white plane generation module 42 then generatesthe white bit plane based on the content of the red, green and blue bitplanes, as discussed below with reference to FIG. 5, wherein the timingof switching the white and color DMDs is related so as to preventunpredictable color and intensity variations and image artifacts.

A person of skill in the art will understand that the white DMD 10′ maybe placed before the colored DMDs 10, as illustrated, or may be placedafter the colored DMDs 10.

As shown in FIGS. 5 and 6, the bit plane data for the white DMD 10′ iscreated within white plane generation module 42 by mixing the R, B and Gbinary bit plane data output from respective converters 34R, 34G and34B, via an OR gate 53 so as to generate a white bit plane that ensuresthe white DMD 10′ is on whenever any one of the red, green or blue DMDs10 is on. However, wherever all of the colors are dark, the white DMDalso turns off, producing a significantly darker dark.

Although not illustrated in FIG. 5, the R, B and G binary bit plane dataoutput from respective converters 34R, 34G and 34B are also sent torespective frame buffers 36 for application to the color DMDs 10, in aconventional manner.

From the foregoing, it will be noted that all ‘ON’ state light from anycolor passes through the OR gate 53 without modification. This is incontrast with using LCOS or LCD for the additional light valve, whichdim all the light for a pixel during the frame, and therefore requirethat the color data be amplified to exactly compensate for the dimmingcaused by the white light value.

With reference to FIG. 6, it should be noted that there is no need toprecisely match the boundaries for the pixels in the white planes withthe pixels in the R, G or B planes, so long as the area of white pixelsfully contains all of the color pixels. If the white area is wider thanthe R, G or B planes by only a small amount (e.g. a few pixels), theonly effect is a small loss of contrast around the edges of brighterareas.

FIG. 7A shows an example of green data for 1 bit plane and the white bitplane it produces. However, since perfect registration between the R, Gand B light valves 10 and the white light valve 10′ is sometimesdifficult or impractical, advantage may be taken of the fact that the‘ON’ pixel areas in the white bit planes can be wider than the imagedata from the color planes. If the white area is expanded by one pixelbeyond the green data then the white bit plane is ‘ON’ for the dark greyarea shown in FIG. 7B. If the white area is expanded by two pixelsbeyond the green data then the white bit plane is ‘ON’ for the combinedlight and dark grey areas shown in FIG. 7B.

According to an embodiment of the invention, expansion of the white areabeyond the green data is performed by logic contained within block 42shown in FIG. 4A, which includes the OR function of block 53 in FIG. 5.Thus, if the OR function 53 generates a white pixel, then block 42generates three expansion pixels (i.e. one to the left and right of thewhite pixel generated by the OR function), and also ensures that thepixels above and below the white pixel are also set. A person of skillin the art will understand that the white pixel data may be expanded bytwo or more neighboring pixels beyond the colored pixel to accommodatemisaligned pixels.

The amount of expansion needed (i.e. one or two pixels) is entirelydetermined by the accuracy of the optical convergence system of theprojector. As discussed above, the only tradeoff in providing such awhite area pixel expansion is a small loss of contrast around the edgesof brighter areas.

Although it may be possible to align all DMDs 10 and 10′ square to eachother so that the image presented to each is a rectangle, if such is notpossible the white image may have a keystone correction applied to it.

If the white DMD 10′ is positioned before the R, G and B DMDs 10 in theoptical path, then keystone correction need only be applied to the whitedata and, and since the white DMD 10′ provides only a gating functionthe applied keystone correction only needs to be a geometry correction(see module 63 in FIG. 8). However, if the white DMD 10′ is placed afterthe color DMDs 10 and the light on the color DMDs 10 is not of uniformintensity, the correction must be applied to the color data prior to thecreation of the white data, using intensity correction modules 61 asshown in FIG. 8.

Preferable, the white DMD 10′ is of the same resolution as the otherDMDs 10, in order to provide maximum benefit with the best contrastaround the image edges. Sharp focus is also an advantage as it reducesthe amount of white overspill required. However, a sharp focus also hasthe potential of creating a moiré pattern as a result of aliasingbetween the pixel grid patterns on the white panel and any one of theother panels, in which case the white DMD 10′ may have to be slightlydefocused.

Segmented displays may be used to increase the effective bandwidth andto reduce some artifacts, wherein segments are located within a singleDMD panel and data for one bit plane is displayed on one segment at thesame time that data from a different bit plane is displayed on anothersegment, as shown in FIG. 9A, which is a simplified depiction of a 4kpanel (only 8 segments are shown for clarity, whereas an actual panelmay have 32 segments). A person of skill in the art will understand thatthe vertically arranged segments shown in FIG. 9A may also be arrangedhorizontally, and that the number of segments may also vary.

Different panels may be separated into different arrangements ofdifferent numbers of bit planes, as shown in FIG. 9B.

Each segment has a different reset signal that causes the loaded bitplane to be displayed. This allows the start and end of each bit planein a segment to occur at different times in different segments. Eachnumbered block in FIG. 9B represents the time sequence that a single bitplane in a single segment is displayed. The numbers inside the blocksshow the order in which the bit planes are written into the DMD 10.

When a pixel from the edge of one segment is misaligned such that itlines up with the white pixel from the neighboring segment, the bitplanes will no longer align and the pixel will not be displayedcorrectly. Although there are several solutions to this problem, theoptimal solution is to output auxiliary timing bits for all bitsequences to identify when a bit plane is activated, which segment it isfor, and which bit plane it is. This provides sufficient data todetermine exactly which bit planes, in adjacent segments, overlap intime. The data for all relevant bit planes in the border area are thenlogically OR'd together, as discussed above.

The bit sequences operate on the output side of the frame buffer 36 andhave different and delayed timing from the input. The auxiliary datamust therefore be captured on the output side of the frame buffer 36 andpassed to the input through an additional buffer (not shown). The datawill therefore not be available for the first frame after a new sequenceis loaded, in which case a minimal number of pixels on either edge of asegment and for the white bit plane are caused to remain on for thefirst frame. This results in all pixels being displayed correctly, butwith a minor image artifact in the form of a possibly visible light linein the image on very dark scenes for one frame.

In the basic implementation the output of 3 color panels feeds into asingle contrast DMD (white) which enhances the contrast of all colors.In an alternate implementation each of the 3 colors can have its owncontrast DMD. In this case the 3 colors do not get OR'ed, but all theother processing involving expanding the bit area for convergenceerrors, and segmented input, still applies.

Also, in alternate implementation each of the three colors (R, G and B)may be provided with its own contrast DMD, in which case the threecolors do not get OR'ed, together although the processing discussedabove takes place for white expansion, segmented display handling, andbit plane timing.

A person of skill in the art may conceive of other embodiments andvariations. For example, it is possible to replace the white DMD 10′with three color DMDs 10, as follows: two red+two green+two blue,resulting in better heat handling, a simpler algorithm, but higher cost.This and other embodiments are believed to fall within the scope of theclaims appended hereto.

What is claimed is:
 1. A light engine for a projection system,comprising: a color splitting-converging prism having a plurality ofcolor sub-assemblies and respective color light valves; at least oneadditional light valve in series with at least one of said colorsub-assemblies and respective color light valves; and a module forconverting source pixel data to color bit planes and generating afurther bit plane from said color bit planes, said color bit planesdriving said respective color light valves and said further bit planedriving said additional light valve, wherein the timing of switching theadditional light valve and respective color light valves is related soas to prevent unpredictable color and intensity variations and imageartifacts, wherein said module includes an OR gate for combining saidcolor bit planes and outputting a white bit plane such that saidadditional light valve is on whenever any of said respective color lightvalves is on and said additional light valve is off whenever all of saidrespective color light valves are off.
 2. The light engine of claim 1,further comprising an intensity correction module for adjusting theintensity of each color to uniform intensity prior to converting saidsource pixel data to color bit planes.
 3. The light engine of claim 1,further comprising a geometry correction module for keystone correctionof said white bit plane.
 4. The light engine of claim 1, wherein saidadditional light valve and said respective colored light valves aredigital micromirror devices.
 5. The light engine of claim 4, whereineach said digital micromirror device is segmented such that data for onebit plane is displayed on one segment at the same time that data from adifferent bit plane is displayed on another segment.
 6. The light engineof claim 5, wherein each segment is arranged horizontally.
 7. The lightengine of claim 5, wherein each segment is arranged vertically.
 8. Thelight engine of claim 1, comprising three additional light valves forenhancing contrast of respective ones of said color light valves.
 9. Thelight engine of claim 1, wherein said additional light valve is locatedin the optical path before said color light valves.
 10. The light engineof claim 1, wherein said additional light valve is located in theoptical path after said color light valves.
 11. A method for increasingcontrast of a projected image, comprising: converting source pixel datato color bit planes; driving respective color light valves with saidcolor bit planes for generating said projected image; generating afurther bit plane from said color bit planes; and driving at least oneadditional light valve with said further bit plane for enhancingcontrast of said projected image while preventing unpredictable colorand intensity variations and image artifacts, wherein generation of saidfurther bit plane comprises combining said color bit planes andoutputting a white bit plane such that said additional light valve is onwhenever any of said respective color light valves is on and saidadditional light valve is off whenever all of said respective colorlight valves are off.
 12. The method of claim 11, further comprisingadjusting the intensity of each color to uniform intensity prior toconverting said source pixel data to color bit planes.
 13. The method ofclaim 12, further comprising keystone correction of said white bitplane.
 14. The method of claim 11, further comprising generating atleast one white pixel on each side of each colored pixel of saidprojected image.
 15. The method of claim 11, further comprisingsegmenting data applied to each of said light valves, such that one bitplane is displayed on one segment at the same time that data from adifferent bit plane is displayed on another segment.
 16. The method ofclaim 15, wherein each segment is loaded with a bit plane under controlof a timing signal that allows the start and end of each bit plane in asegment to occur at different times in different segments.